What I like about wealth redistribution scenarios is how they're so much easier to sell when you ignore the potential negative effects.
It's like playing monopoly and someone wins all the money. The game is over at that point. Now sure... some people try to keep it going by lending money. But when the winner gets all that as well and the other party eventually has mortgaged and sold everything there is no more money for the winner to get.
Getting money out of the hands of accumulators and hoarders (oh, I mean, "investors") and back into the hands of consumers would be the best thing that could happen to everyone at this point in time. It would re-energize the system. But... give that the hoarders have much more political clout this is difficult to do. Even if it is ultimately in their best interest.
The banking system may be seriously flawed, but I'm not following your critique of accumulators / hoarders / investors. When people acquire excess money, they rarely put in under a mattress. They don't need it right now so they enter into a transaction with someone who does need it in return for more money later.
For example, when a "hoarder" puts money into a bank account, the bank doesn't just stare at it. It lends it to someone who needs money now. The bank gets paid back interest and some of that goes to pay interest to the "hoarder". Obviously, this is a simplification, but the idea is the bank is an intermediary that matches people who want more money later with people who want money now.
People keep wealth in other forms besides banks.
There is a lot of hoarding going on and it is one of the primary reasons the economy is not growing.
But the point is... as wealth concentrates, consumers lose the ability to purchase. So things grind to a halt. There has to be some mechanism to counter this trend.
Where do people keep money that involves the money truly sitting inactive?
There are any number of places.
Besides, just because money is in a bank doesn't mean it is actively being lent out. And even if it is, that isn't really relevant to the original point anyway.
Concentration of wealth is bad for a consumer driven economy.